Load Commutated Inverter (LCI) Synchronous Motor Drives using a thyristor inverter bridge do not need forced commutation means, because automatic thyristor turn-off is achieved with a synchronous motor as the load, if it has a leading phase angle with respect to the load voltage. For a given load, increasing sufficiently the field will produce such leading power factor. See, for instance:
"The Synchronous Machine as a Self-Controlled Converter-Fed Motor" by Dieter Kollensperger in Siemens Review XXXV (1968) No. 5, pp. 195-201; and
U.S. Pat. No. 4,713,743 of Dec. 15, 1987 (Alberto Abbondanti).
With an induction motor, however, this possibility no longer exists. The load power factor is lagging for all machine excitation levels. Therefore, specific circuitry must be used to allow a leading phase angle to take place, thereby providing natural commutation of the inverter bridge. To this effect, a general solution is to add a large capacitor bank in parallel with the motor, so that the lagging load power factor be overcompensated, the net result being that leading VAR's are supplied to the composite load. Accordingly, the resultant leading power factor angle will insure natural commutation of the inverter poles. In such case, the motor drive is referred to as a Load-Commutated inverter Induction Machine (LCI/IM) drive. Thus, when the power factor of the inductor motor has been overcompensated to produce a leading load power factor to a current-source, in principle the induction motor behaves much like a synchronous motor as far as current-source inverter (CSI) is concerned, for a significant speed range.
However, the size of the capacitor bank has to be taken into account, especially in view of the speed range of the drive. The torque may vary with the square of the speed, while the load-speed profile should accommodate 100% torque at 100% speed. Increasing the speed range requires increasing the capacitance added in parallel. The demand for more capacitance may also come from the use of reactors to insure a limited range of the current during commutation and protect the thyristors. When reducing the speed of the motor, the added capacitance, even in excess, will improve the effectiveness of the load commutation process, however, not all the way. At low speed the "commutation lead time" due to the capacitors in parallel quickly drops below any adopted safe limit. Commutation at low and even very low speed is necessary, if only because the motor drive must be brought up to speed from the start. Therefore, like with a synchronous motor drive, there is a need in LCI/IM systems for an inverter forced-commutation arrangement allowing start-up. Since such commutation function is not required beyond the low end of the speed range, when the motor voltage is moderate and the switching frequency is reduced, it must be devised at low cost and with a limited performance capability.
Prior art techniques, other than natural commutation, for commutating an inverter may be typically seen in:
"A Circuit for Two-Stage Artificial Commutation of an Inverter" by V. P. Bakharerskii and A. M. Utevski in Direct Current, June 1957, pp. 153-159; and
"Analysis of a Novel Forced-Commutation Starting Scheme for a Load-Commutated Synchronous Motor Drive" by Robert L. Steigerwald and Thomas A. Lipo in IEEE Trans. IA-15 Jan./Feb. 1979, pp. 14-24.